Os"mi*um (?), n. [Gr. a smell, odor, akin to to smell. So named in allusion to the strong chlorinelike odor of osmic tetroxide. See Odor.] Chem.

A rare metallic element of the platinum group, found native as an alloy in platinum ore, and in iridosmine. It is a hard, infusible, bluish or grayish white metal, and the heaviest substance known. Its tetroxide is used in histological experiments to stain tissues. Symbol Os. Atomic weight 191.1. Specific gravity 22.477.